In general, an integrated circuit refers to an electrical circuit contained on a single monolithic chip containing active and passive circuit elements. Integrated circuits are fabricated by diffusing and depositing successive layers of various materials in a preselected pattern on a substrate. The materials can include semiconductive materials such as silicon, conductive materials such as metals, and low dielectric materials such as silicon dioxide. The semiconductive materials contained in integrated circuit chips are used to form ordinary electronic circuit elements, such as resistors, capacitors, diodes, and transistors.
Integrated circuits are extensively used in electronic devices, such as digital computers, because of their small size, low power consumption, and high reliability. The complexity of integrated circuits range from simple logic gates and memory units to large arrays capable of complete video, audio and print data processing. Presently, however, there is a demand for integrated circuit chips to accomplish more tasks in a smaller space while having even lower operating voltage requirements.
As stated above, integrated circuit chips are manufactured by successively depositing layers of different materials on a substrate. Typically, the substrate is made from a thin slice or wafer of n-type or p-type silicon. The active and passive components of the integrated circuit are built within a thin n-type epitaxial layer on top of the substrate. The components of the integrated circuit can include layers of different conductive materials such as metals and semiconductive materials surrounded by low dielectric insulator materials. In attempting to improve integrated circuit chips, attention has been focused upon not only using different materials to construct the chips but also upon discovering new processes for depositing the various layers of materials on the substrate.
The materials that are deposited in layers on semiconductor wafers generally need to be placed onto the wafer according to a predetermined pattern. In order to place the materials on the wafer according to a pattern, typically a coating is first formed on the wafer. Channels and pathways are then formed into the coating which provide the pattern for applying the materials that are used to form the integrated circuit. Applying the precursor coating on semiconductor wafers as described above is generally done through a lithographic process. Typically, five to twenty complete lithographic operations are required on each wafer depending on the type of integrated circuit. For most applications, the precursor coating applied to the wafer is made from a photoresist material, which is a photoreactive material whose properties are altered by optical radiation over a specified wavelength range. Due to the photoreactive properties of photoresist materials, patterns can be formed into the coatings using light energy.
For instance, in one embodiment, first the semiconductor wafer is coated with a photoresist material. The coating of the photoresist material should be uniform and should be highly adherent to the wafer. After the coating is applied to the wafer, the coating is typically baked at a temperature of up to about 100.degree. C. in order to remove any solvents that may be contained within the coating.
The next step in the process is to form channels and pathways into the photoresist coating. This is done by first aligning the wafer with a mask that is printed with a predetermined pattern. For instance, the mask can be a gelatin photographic emulsion on a glass plate that is placed over the semiconductor wafer. The wafer is then exposed to light energy that reacts with the photoresist material. In particular, since the mask is placed over the wafer, the light energy being directed onto the wafer only contacts the photoresist coating according to the pattern that has been printed onto the mask.
After being exposed to light at a particular wavelength, if desired, the wafer can then be baked once again. After being baked, the wafer is rinsed in an appropriate solvent. Depending upon the type of photoresist material used, the solvent either removes the photoresist material that has been exposed to light or removes the photoresist material that has not been exposed to light. If the solvent removes the exposed portion of the photoresist coating, the photoresist material is considered a "positive" photoresist. If the solvent, on the other hand, removes the unexposed regions without effecting the exposed regions, the photoresist material is referred to as a "negative" photoresist. Regardless, once exposed to the solvent, portions of the photoresist coating are removed leaving behind a desired pattern.
Depending upon the particular application, the wafer can then be etched in order to remove those parts of the underlying film that are not covered by the photoresist. Etching results in the formation of windows in the mask film. Etching is performed in order to connect a layer to be deposited on a semiconductor wafer with a layer that has been deposited previously.
After etching, the next layer of the semiconductor wafer can be constructed. As mentioned above, the layer being deposited onto the patterned photoresist coating can be a semiconductive material, a conductive material, or a low dielectric material.
Thus, as described above, the lithographic process typically comprises the steps of coating a wafer with a photoresist and then taking a suitable patterned mask and imaging it onto the surface of the precoating. An engraving process is then performed where a pattern is formed into the photoresist coating. The pattern is used for opening windows in an underlying layer to define semiconductor regions and/or to remove metal from a coated wafer in order to delineate a pattern of interconnections.
In the past, in order to form a photoresist coating on a wafer, the photoresist materials have been spin coated onto the substrates.
According to this process, the substrate is first placed on a vacuum chuck and rotated at high speeds. A solution containing the photoresist is then applied to the substrate. Due to centrifugal force, a coating is formed on the substrate which is dried and annealed on a hot plate or in a furnace.
Difficulties, however, have been encountered with this process in getting the photoresist to adhere to the substrate and in producing a film having good physical properties. Such deposition methods usually generates course material with undesirable physical properties. Further, other problems have also been experienced in controlling various parameters, such as the thickness and uniformity of the film especially when applied to larger wafers.
In view of the above deficiencies of the prior art, a need exists for a process for depositing photoresists on substrates for use in integrated circuit chips. In particular, a need exists for a process for uniformly depositing photoresists on substrates, such as silicon, during the fabrication of integrated circuits.